Hello! Anyone know the actual level of the internal calibration test tone on the 990? I mean, if I put the master level to 0dB, the tone that is output must somewhat be equivalent to an actual level on a digital media, say a DVD.

I seem to recall that these tones are usually at -20dBfs or -30 dBfs.

Thanks!

I don't know what it is, but it can be determined pretty easily. However, bear in mind that the test signals are not really tones - they are band-limited random noise. I mention this because in order to get an accurate reading (for example, in terms of RMS volts), you would need a meter that is true RMS. Many 'budget' volt meters are desigend with measuring a simple, single sinusoid in mind (i.e. 50 or 60 Hz as those are the mains frequencies around the world), and most will not accurately measure something like band-limited random noise.

Having said that, and assuming that you had such a meter, then you could measure this for yourself, but you would need a reference file against which you could compare the observed values, and the reference file would also have to be known in terms of its scale (i.e. how many dBFS).

So, let's suppose that you had a 60 Hz wav file that was 0 dBFS (all bits hi). You could play this through the 990 (with amplifiers disconnected) and for a given setting on the volume control, observe its value in RMS volts.

Since I don't know that actual scaling of the 990's outputs, I can't say (or guess) what you would see with the volume control set to 0 dB (although I think you can only get to -1 dB, but I am not sure about that. Anyway, you could play the 60 Hz wav file and note its voltage. Then, you could turn on the test tones and measure their RMS voltage. Since you know the reference file is full scale, then the RMS value observed for the band-limited random noise could be used to compute how many dB down the test signal (random noise) is.

For example, suppose the 60 Hz wav file (for a given setting on the volume control - and you DO NOT change the volume control setting for your measurements) produced 500 mV RMS. For that SAME setting on the volume control you measure the test signals (band-limited random noise) and observe that 330 mV RMS are produced. Given these two values you could make the calculation for dB relative to Volts. That is:

dB = 20 * log (V1/V2)

substituting, we get

dB = 20 * log (330/500) --> dB = 20 * log (0.66) --> dB = 20 (- 0.180) --> dB = -3.61

That is, based on the observed voltages (using the control signal (0 dBFS) as your reference) you would know that the test signal (band limited random noise) was 3.6 dB down from full scale, or -3.61 dBFS. You could say this because the file against which you are comparing the band-limited noise is known to be a full-scale file.

Assuming that the D/A has high linearity then this relationship should hold for any 'reasonable' volume control setting. In other words, whether you did this experiment with the volume control set to -30, -20, -5, or 0 (assuming the volume control can reach 0 dB) the RELATIVE difference observed in the two signals' RMS values should be the same.

So, let's assume that the D/A is very, very linear, and you conduct this experiment at several volume control settings..., write them down, and compute the difference in their magnitudes, in dB. Now...the values that I am showing below were pulled out of the air for the sake of illustration, but pay attention to the difference, in dB - it is conserved as a function of the volume control setting on the 990:

-10 dB SETTING
Vref file: 500 mV, Vtest tone:330 mV, delta dB = 3.61

-13 dB SETTING
Vref file: 353.5 mV, Vtest tone:233 mV, delta dB = 3.61

-16 dB SETTING
Vref file: 250 mV, Vtest tone:165 mV, delta dB = 3.61

-20 dB SETTING
Vref file: 50 mV, Vtest tone:33 mV, delta dB = 3.61

-30 dB SETTING
Vref file: 15.8 mV, Vtest tone:10.44 mV, delta dB = 3.61

-40 dB SETTING
Vref file: 5 mV, Vtest tone:3.3 mV, delta dB = 3.61

If you were to do this over the full range of the volume control, at some point (most likely, as the volume control's setting went further and further negative), the values that you would observe between the two files would no longer 'track', and you would start to see the delta change from (in this example) 3.61 dB to something otehr than that, and this is because of non-linearity in the D/A as well as other elements in the circuit topology that could affect linearity.

Likewise, if you did this experiment at (assuming that you can get there) 0 dB on the 990's volume control, you should see this same relationship. Again, I am NOT saying that the values that I have shown in my example are what you would observe as actual voltages - they are only to illustrate my point and methodology.

So far so good, however, keep in mind that getting an accurate RMS estimate of these values depends upon that which you are using to measure them. A more appropriate way would be to compute the signal power using Fourier analysis, and using the same settings for each analysis. That is, PC-based spectrum analyzers that have a highly linear A/D front end for the measurement and the requisite signal processing software to do these calculations would be helpful.

Using such an animal, you could either calibrate it with a known reference voltage (to get dB relative to a known reference, and in so doing, also be able to know the actual voltage of the signals measured), or just rely upon the relative difference in the values.

For example, if you used 1 VRMS as a calibration signal for the analyzer, then you would know that in the ANALYZER that 0dB would equate to 1.0 VRMS; were you to choose a calibration file that was 100 mV RMS then 100 mV RMS would be your reference, and as such, when the analyzer measured 100 mV RMS it would show up on the analyzer as 0 dB.

Thus, if you were to calibrate the analyzer using your 0 dBFS file for the HIGHEST volume control setting (i.e. 0 dB on the 990), then your first measurement would show that the reference file was indeed 0 dB, and when you play the test tones, the number of dB down (from full scale) would be calculated for you, and this would be the number of dB down from full scale the test tones are.

Likewise, as you work your way down the volume control (going more negative) then there would just be an offset to the values you see in the analyzer, expressed in dB. For example, if you calibrated the analyzer with the 990's volume control at 0 dB, then when you measured the full-scale 60 Hz wav file at a volume control setting of - 10 dB, this would be its magnitude in the software analysis (i.e. - 10 dB), but again, the delta you observe (when you play the test tones) should be the same DELTA as when you took the measurement with the 990's volume control at 0 dB - there would only be an offset.

It doesn't really matter what value you use as a reference (unless you want to know the actual value in volts...and then it matters), but as long as you do not alter any gain settings, the RELATIVE differences that you measure, in dB will be correct.

Does this help?

Call Outlaw and ask Scott. He can either tell you or find it out for you. I though it was at 75 db when at 0 on the dial.

What you're referring to is the C-weighted SPL observed at the listening point as relayed through the speakers. The question that was asked was how many dB down from full-scale is the magnitude of the band-limited random noise (the 'test tones'); the two things are related, but not the same.

For example, you could have a 0 dBFS file (i.e. all bits hi) being played back through an audio system, but the SPL is a function of the volume control on the 990, the amplifier gain, and the speaker sensitivity. For example, a full-scale file could be played back at 45, 57, 78, 80...or whatever dB SPL, but the file itself remains full-scale.

What he's really asking is when the tones are generated, how many dB down from full-scale is the magnitude of that file. Remember that the value in dB relative to full scale is not a function of the sound pressure level achieved - it's a function of the file itself.

For example, if the magnitude of file "A" is full scale (0 dBFS) and the magnitude of file "B" is 70.7% of the value of file "A", then file "B" is - 3 dBFS. Likewise, if the magnitude of File "C" is 50% that of file "A", then we know that the magnitude of File "C" is - 6 dBFS. We can say this because we know that file "A" is full-scaled, that is, 0 dB.

That's why I suggested the measurement approach that I did, because comparing the magnitude of an unknown file (the 'test tones') to the magnitude of a known file (the full-scale file) for a given volume setting on the 990 would tell you how many dB down is the magnitude of the unknown file.

You could make a crude approximation of the value though by using a SLM and playing the full-scale file (for a given SAFE volume setting) noting that value, and then invoking the tones and noting the SPL observed for the band-limited noise. It's not as accurate as a measurement of the signal coming from the 990 (because the speaker-based approach relies upon speakers, and is influenced by the room, its acoustics and so on - and therefore where you measure affects what you measure; these caveats do not apply when the measurements are made driectly from the preamp's outputs).

Still, if you played the known full-scale file at a reasonable volume setting (maybe -40 or - 50 dB on the 990 to start), noted its SPL, then noted the SPL generated for the test tones (again, for the same volume setting on the 990), the DELTA in SPL would be a crude approximation of the scaling of the test tones relative to full scale (because speaker SPL is proportional to applied voltage, not power).

In other words, if the fullscale file, for a given volume setting on the 990 yielded 87 dB SPL, and the test tones yielded 58 dB(same volume setting on the 990 and the measurement mic in the same location used to measure the full-scale file's SPL), the you could say that the test tones are approximately 87 - 58 = 29 dB, or, 29 dB down from full-scale, or written another way, -29 dBFS.

Again, using the electrical signals (preamp outputs) is a more reliable method though as it eliminates all the influences of room modes, overall acoustics, speaker sensitivity / directivity / frequency response, and so on.

I don't see him asking the question you answered. I think he is asking how the test tone compares to a real world signal. The tone may ne more or less than say a CD or DVD players output. Since most of us don't have the type of test gear you must be used to and we almost all have a telephone his best bet is to call or IM Outlaw directly.

Here's what he said:

"Hello! Anyone know the actual level of the internal calibration test tone on the 990? I mean, if I put the master level to 0dB, the tone that is output must somewhat be equivalent to an actual level on a digital media, say a DVD.

I seem to recall that these tones are usually at -20dBfs or -30 dBfs.

Thanks!"

The fact that he mentions dBFS (decibels relative to full-scale) is the terminology that is used to describe the magnitude of a digital file. Also, that he cites the two values as negative (-20 dBFS and -30 dBFS) makes sense; in a digital file, 0 dB equals full-scale, that is, all bits hi. Therfore, anything that is less than full scale will be, by definition, signed as negative.

Therefore, as I interpret it, he's asking about the magnitude of the test tones as if to say "are they (the sett-up test tones generated by the 990) of similar magnitude as what would be found in other media".

The level that is reproduced from an audio system, using a digital file as its input, depends upon the scale of the file itself (i.e. how many dBFS), the gain of the preamplifier, the gain of the power amplifier, and the sensitivity of the speakers (as well as the room in which they are placed and the acoustics thereof), and where the pressure is measured. Thus, the level of the test tones cannot be stated in dB SPL (that is, dB referenced to 20 uPa) because it's meaningless - it only makes sense to describe them in terms of dB relative to full-scale. The 75 dB(C) value that Outlaw cites in the manual is the dB SPL value suggested such that it will have a relatively high signal to noise ratio (in a typical home), as well as being easily heard, yet not offensively loud.

Again, the "actual level" (as he put it) can only be expressed in dB relative to full scale, because that's the only scale that has any relevance in a digital file.

Regardless, it is still easier for him to call Outlaw and ask for the data anyway he wants it. Since the 990 doesn't do room correction the actual level compared to other devices is not much value in reality. The test tone seems to be loud enough for the system to "hear" it and get a good idea how far away the speakers are and what efficiency they are capable of. Other than that the system doesn't do much else compared to something like Audyssey or Trinnov.

You're right that it is easier for him to call Outlaw - I have no disagreement with you on that as that is something that should be at the fingertips of Engineering.

My posts were intended to give him (or anyone else out there who might want to know how they could go about figuring it out for themselves) the means by which the dBFS value of the test signals could be determined.

Hello! Thanks Old school, but what I need to know is really simpler than your detailed answer!

I want to link the test tone output to an equivalent level on a digital media. The goal is to be able to calibrate the actual dB SPL output of the system to reference level.

For example, if the test tone is equivalent to -20 dBfs on a media, I will calibrate the system to output 85 dB SPL from each channels, which is 20 dB less than the reference peak level of 105 dB SPL.

If the output of the tone is -30 dBfs, I will calibrate to 75 dB SPL, so 0 dBfs is still 105 dB SPL.

I work in pro sound and understand all this really well, I just want to know the test tone equivalent level!

Thanks again!

Hmmm...

Well, again, the simplest thing (as XenonMan pointed out) would be to call Outlaw.

So just to be clear, if Engineering (Outlaw) told you that the test tones were -17 dBFS (hypothetically speaking), you would set it up such that you would calibrate to 105 - 17 = 88 dB SPL. Is that correct? That would be 17 dB less than the peak level.

Correct?

Yes, that's it!

I want my system to reproduce the soundtrack at the same level that it was produced, hence I want it to be calibrated acurately. I really should purchase some Blu-ray test disc with a known level test tone on it!

I just ran a 'quick and dirty' test and came up with -29.7 dBFS. I plan on repeating this later tonight in a more controlled fashion, but the value seems reasonable to me. Once I finish, I'll post the final answer that I get up to my FTP site. The test tones are actually fairly band-limited random noise (I think from around 100 Hz to 2 kHz, but again, I'll go back and check).

I'm curious to read what the actual specified level of the tones is (in dBFS).

[Update: OK, so I repeated it a second time, and I got the same answer. As to how I came up with the value, here is what I did:

1) Construct a wav file that is 70.7% of full scale, that is, - 3 dBFS.
2) Connect a digital recorder to the front balanced outputs on the 990.
3) Select an arbitrary volume level on the 990. I chose - 20 dB
4) Play the known - 3 dBFS wav file from my Audiotron taking its optical output into the 990.
5) Set the gain on the digital recorder such that when playing the known - 3 dBFS file, no overload (or under-range) occurs on the digital recorder.
6) Acquire (record) 30 seconds of the - 3 dBFS file.
7) Invoke the menu on the 990 and go into the channel calibrate menu. Select Manual.
8) Acquire (record) the left front channel output.
9) Acquire (record) the right front channel output.
10) Take the SDRAM card from the digital recorder to my PC and edit the files for length to 15 seconds' length.
11) Open the wav file of the known - 3dBFS file in Spectra Plus (FFT Analysis software).
12) Select dBV (decbels relative to volts) and calibrate using the known - 3 dBFS file.
13) Post Process to compute overall RMS power. Result: - 3 dB
14) Post-process the wav file uisng flat-top windowing and 16384 lines in the FFT with 95% overlap.
15) Post Process to compute the overall RMS power of the test tone file using the same calibration factor as was used for the - 3 dBFS file. Result: - 29.97 dB

Note that Spectra Plus does not allow units of FS (full scale), however, the math behind dBFS and dBV is identical - the only difference is that for dBV the reference is 1.0. However, both follow the general expression used for dB, namely dB = 20 * log (v/vref).

If you want to see plots detailing these results, I have posted to .png files on my FTP site in a folder called test_tones_magnitude. To log on to the FTP site (from a browser), just type (or copy and paste the string below into a browser):

FTP://owners_990.immersifi.com:combfilter1.immersifi.com (Sorry!... I mistyped that... the correct link to copy into your browser is as follows:

FTP://owners_990.immersifi.com:combfilter1@immersifi.com - my bad).

If you are using an FTP client, then the logon credentials are:

User: owners_990.immersifi.com
Password: combfilter1
Host: immersifi.com

There are a bunch of other directories there, but the plots are in the test_tones_magnitude folder.
Anyway, it does indeed seem to be the case that the tones on the 990 (the total power in them) is essentially - 30 dBFS.]

Hello, sorry for the late reply!

The -30 dB figure is perfectly logical, as I have seen it mentionned elsewhere. I actually think it may be a Dolby standard.

Thanks for confirming!

You're welcome; 'glad that I could be of assistance.

The whole 'trick' in determining the unknown value of the test signal was in comparing a file having a known dBFS value to the test signals produced by the 990 (for a given gain setting on the 990's volume control).

Having the (FFT) spectrum analyzer available simplified things because it made it possible to average each signal in the same way. That is, were one to try this using only a true RMS DMM (in order to compare the RMS values of the two files), the answer obtained for the value of the test signals would be varying (on the DMM display) due to the differeing crest factor as a function of time; this would make estimating its value somewhat of a crapshoot - you would be in the ballpark, but it would still only be a 'guesstimate'. For the signal that I created (a sine function), this isn't really an issue as such a signal has uniform crest factor as a function of time, so the DMM's display would always show the same RMS voltage.